The invention relates in general to static AC motor drives and more particularly to an improved and low cost design for small rating motor drives of this type.
Static AC motor drives typically draw power from a DC source coupled with an inverter. The prior art is replete with techniques for controlling the static switches of the inverter in order to control current supply, e.g. the torque of the motor and the frequency of the inverter, e.g. the speed of the motor. In particular special circuits have been designed which in the starting mode insure that the flux in the motor is sufficient while preventing an inrush of starting currents which would overload the inverter.
In general, it is not economically feasible to supply starting inrush currents to an induction motor when it is excited by a variable frequency, solid-state inverter. If the output current of the inverter is limited to about one per unit by appropriately controlling the inverter output voltage and if the motor is operating at a slip greater than its rated pull out slip, the torque generated by the motor will be quite meager and may be insufficient to accelerate the motor to the desired speed. Such a condition could occur during starting or if the motor were temporarily overloaded. In order to prevent this, the usual practice is to measure the motor speed, calculate the motor slip, and limit the slip to less than pull out slip by adjusting the excitation frequency. Because of the tachometer cost, this approach is not feasible for a low cost drive.
The object of the present invention is to provide a low cost, variable speed, constant torque induction motor drive.